Myocardial ischemia is a serious medical condition resulting from inadequate blood supply to the myocardium, a muscle of the heart that alternately contracts to permit the heart's ventricular chambers to pump blood, and then relaxes to permit the heart's chambers to receive blood. Ischemia is characterized by abnormally low myocardial blood perfusion; that is, abnormally low blood flow to myocardial tissue. An ischemic event can be triggered by a rupture of a plaque in a coronary artery, an event that may generate a clot causing the artery to become occluded and prevent oxygenated blood from reaching myocardial tissue. When this occurs, a patient may experience chest pain, but may discount the sensation as an aberration or assume that the pain is related to another condition, such as arthritis, for example. Other times, the patient may not feel any pain. Within seconds of an ischemic event, myocardium contractility can become impaired as the myocardial tissue becomes starved of oxygen, resulting from the lack of blood flow due to the occlusion. Within fifteen to twenty minutes of the event, heart damage can occur if the blockage is not removed, and the longer the heart muscle is deprived of oxygen, the greater the risk of irreversible muscle damage, which may occur after a few hours. Prompt detection of an ischemic event is therefore desirable, and may allow timely therapeutic interventions, such as various thrombolytic or angioplastic therapies designed to clear the occluding material and restore free blood flow to the myocardial tissue.
One known method of detecting ischemia is to obtain an electrocardiogram (ECG) of a patient's coronary electrical activity, and analyze the S-T segment of the ECG for deviations. The S-T segment is the portion of the ECG signal between the S wave and the T wave, and is known to exhibit variations following an ischemic event. Detecting an S-T segment deviation may indicate ischemia. An ECG may be obtained using twelve leads placed in various locations externally on the skin of the patient to sense coronary electrical activity in a manner known to those skilled in the art. Another method of obtaining an ECG uses electrodes attached to an implanted medical device, such as a pacemaker or defibrillator, implanted in a chest region of a patient to sense cardiac electrical activity. The S-T segment of this ECG can then be analyzed for deviations in the manner described above.
Similarly, pulmonary edema is also a serious medical condition. Pulmonary edema is characterized by an excess accumulation of fluid within a patient's lungs, and can be an indicator of cardiac-related diseases, such as heart failure. It is possible to detect fluid in the lungs by making an electrical impedance measurement across the lungs. The more fluid present in the lungs, the lower the impedance. One known way of making such an electrical impedance measurement is to use an implanted medical device such as a pacemaker or defibrillator. An electrical impedance measurement is conventionally made between right ventricular chamber electrodes connected by a lead to the implanted medical device, and another electrode at the implanted medical device itself. As such, the impedance measurement samples thoracic tissues, including the lungs.
U.S. patent application Ser. No. 10/303,305, filed on Nov. 25, 2002, by Andres Belalcazar and Robert Patterson (the present inventors), and Rebecca Shult, describes another way of measuring lung impedance using an implantable medical device to make an electrical impedance measurement between an electrode positioned epicardially over the left ventricular wall of the heart and connected to the implantable medical device, and another electrode at the implantable medical device itself. The impedance is measured by applying an electrical stimulus current to the implanted electrodes and measuring the resulting voltage using other implanted electrodes, and then calculating the ratio of voltage to current. It is also possible to detect fluid changes in the lungs by making an electrical impedance measurement using only external electrodes attached to the skin.